Fabrication method of nitride light emitting diodes

ABSTRACT

A fabrication method of a nitride semiconductor LED includes, an Al x In y Ga 1-x-y N material layer is deposited by CVD between an AlN thin film layer by PVD and a gallium nitride series layer by CVD, to reduce the stress effect between the AlN thin film layer and the nitride layer, improve the overall quality of the LED and efficiency. An AlN thin film layer is deposited on a patterned substrate having a larger depth by PVD, and a thin nitrogen epitaxial layer is deposited on the AlN thin film layer by CVD, which reduces the stress by reducing the thickness of the epitaxial layer and improves warpage of the wafer and electric uniformity of the single wafer; the light extraction efficiency is improved by using the large depth patterned substrate; further, the doping of high-concentration impurity in the active layer effectively reduces voltage characteristics without affecting leakage, thereby improving the overall yield.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims priority to,PCT/CN2015/073465 filed on Mar. 2, 2015, which in turn claims priorityto Chinese Patent Application No. 201410354965.0 filed on Jul. 24, 2014;and PCT/CN2015/078638 filed on May 11, 2015, which in turn claimspriority to Chinese Patent Application No. 201410549383.8 filed on Oct.17, 2014. The disclosures of these applications are hereby incorporatedby reference in their entirety.

BACKGROUND

Physical vapor deposition (PVD) refers to a technology that under avacuum condition, material source—the solid or liquid surface—isgasified into gaseous atoms or molecules, or part of them is ionizedinto ions, and then through the low-pressure gas (or plasma) process, aspecial-purpose film is deposited on the substrate surface. The physicalvapor deposition method mainly includes vacuum vapor deposition,sputtering coating, arc plasma plating, ion plating, molecular beamepitaxy and the like; the method is applied not only in the depositionof metal film, alloy film, but also the deposition of compound film,ceramic film, semiconductor film, polymer film and the like; thetechnology process is simple, has less pollution on the environment, andsaves raw materials, and produces even and dense film with strongadhesion to the substrate.

In view of the above advantages of the PVD method, and with the rapiddevelopment of the light emitting diode (LED) research, the method iswidely applied in the fabrication of light emitting diodes. U.S. PatentApplication Pub. No. 2013/0285065 discloses that the AlN thin film layerformed by the PVD method is flat and its roughness is less than 1 nm;that the lattice quality is good and the 002 peak width at half heightis less than 200; and that on the thin film layer an n-type layer, alight emitting layer and a p-type layer and the like nitride layers aredeposited by chemical vapor deposition (CVD). In the actual fabrication,the deposition of the crystal layer by the chemical vapor depositionmethod is greatly different from the PVD method in terms of the growthchamber environment, and the crystal layer is composed of the GaNsystem, and has large lattice mismatch with the AlN thin film layer,resulting in large stress between the AlN thin film layer by the PVDmethod and the nitride layer by the CVD method, which easily leads topoor quality of light emitting diode and low luminous efficiency.

Further, with the application of the patterned substrate technique, apattern having a fine structure is fabricated on the planar substratesurface, and then the LED material epitaxially grows on the patternedsubstrate surface. The patterned interface changes the growth process ofthe GaN material, and restrains the defects from extending to theepitaxy surface and improves the efficiency of the quantum in thedevice; at the same time, the roughened GaN/substrate interface diffusesthe photons emitted from the active region, so that the originallyfull-emitted photons have the opportunity to exit from the device so toeffectively improve the light extraction efficiency. However, forepitaxially growing the LED material on the patterned substrate surfaceby the present conventional metal organic chemical vapor deposition(MOCVD), the depth of the pattern on the patterned substrate surface isrequired to be less than 2 μm, and if the depth is larger than thisvalue, the MOCVD method cannot result in a quality epitaxial film layer;in addition, due to the characteristics of uneven surface of thepatterned substrate, a thick buffer layer should be laminated in then-type layer and the substrate in the LED structure, causing theepitaxial layer surface before the growth of the n-type layer to achievethe required smooth structure so to be conducive to the subsequentlamination of epitaxial layers; yet the thick underlayer structureproduces a greater stress, resulting in a greater warping of the LEDstructure upon the completion of the growth, which is not conducive tothe follow-up process implementation (such as splinter incurred infollow-up process) and leads to obvious difference of electricproperties in different locations in a single LED structure and furtherthe impact on the growth yield rate; further the doping concentration ofthe active layer is also limited due to the impact of the quality of theunderlayer, therefore higher doping level cannot be obtained, thuslimiting the further improvement of the voltage isoelectricity.

SUMMARY

To solve the above problem, and according to the first aspect of thepresent disclosure, a fabrication method of nitride semiconductor isprovided, wherein, an Al_(x)In_(y)Ga_(1-x-y)N material layer by CVDmethod is deposited between a PVD AlN thin film layer and a CVD nitridelayer, the Al_(x)In_(y)Ga_(1-x-y)N material layer can help reduce thestress effect between the AlN thin film layer and the nitride layer,improve the overall quality of the light emitting diode and finallyimprove the luminous efficiency.

According to some embodiments disclosed herein, a fabrication method ofnitride light emitting diode includes the following steps: step 1:providing a substrate, and depositing an AlN layer on the substrate byphysical vapor deposition (PVD) method to form a first buffer layer;step 2: depositing on the AIN layer an Al_(x)In_(y)Ga_(1-x-y)N (0<x≦1,0≦y≦1) layer by chemical vapor deposition (CVD) method to form a secondbuffer layer; and combining the first buffer layer and the second bufferlayer to form an underlayer; step 3: depositing an n-type galliumnitride layer, a light emitting layer and a p-type gallium nitride layeron the nitride underlayer by the CVD method.

The second buffer layer composed of Al_(x)In_(y)Ga_(1-x-y)N material andformed by the CVD method and the first buffer layer composed of AlNmaterial are both of an aluminum contained material layer, therefore thetwo buffer layers have similar material coefficients and the latticemismatch between the two buffer layers is low; further, due to thedeposition method of the second buffer layer is the same as that of thedeposited layer in step 3, preferably metal-organic chemical vapordeposition (MOCVD) method can be chosen, thereby reducing the stressbetween the materials in step 1 and step 3, and improving the quality ofthe lattice layer of the underlying layer and of the overall epitaxialstructure.

In some embodiments, the thickness of the first buffer layer formedranges from 5 Å to 350 Å.

In some embodiments, the thickness of the second buffer layer formedranges from 5 Å to 1500 Å.

In some embodiments, the growth temperature of the second buffer layerformed ranges from 400° C. to 1150° C.

In some embodiments, the n-type gallium nitride layer formed in step 3is an n-type doped gallium nitride layer or a combined layer of anundoped gallium nitride layer and the n-type gallium nitride layer.

In some embodiments, the underlayer formed is undoped or doped withn-type or p-type impurity.

In some embodiments, the n-type impurity is silicon or tin.

In some embodiments, the p-type impurity is zinc, magnesium, calcium orbarium.

In some embodiments, the concentration of the n-type or p-type impurityranges from 10¹⁷/cm³ to 10²⁰/cm³.

In some embodiments, the first buffer layer is formed by depositing in aPVD chamber; the second buffer layer is formed by depositing in an MOCVDchamber.

In the above method, the second buffer layer composed ofAl_(x)In_(y)Ga_(1-x-y)N material and formed by the CVD method has a lowlattice mismatch with the first buffer layer composed of the AlNmaterial, and the deposition chamber environment of the second bufferlayer is the same as the growth environment of the deposition layer instep 3, and accordingly, the method can reduce the stress between thematerials in step 1 and step 3 and the quality of the whole epitaxialstructure can be improved.

According to the second aspect of the present disclosure, a fabricationmethod of nitride semiconductor is provided, wherein, an AlN thin filmlayer is deposited on a patterned substrate having a larger depth by thePVD method and a thin nitrogen epitaxial layer is deposited on the AlNthin film layer by the CVD method, which reduces the stress by reducingthe thickness of the epitaxial layer and so improves the warpage of theepitaxial wafer and further the electric uniformity of the single wafer;at the same time, the light extraction efficiency is improved by usingthe large depth patterned substrate; further, the doping ofhigh-concentration impurity in the active layer effectively reduces thevoltage characteristics while not affecting the leakage, therebyimproving the overall yield of light emitting diodes.

According to some embodiments, a fabrication method of nitride lightemitting diode includes the following steps: step 1: providing andplacing a substrate in the physical vapor deposition (PVD) chamber; step2: depositing an AlN material layer on the substrate by the PVD method;step 3: taking out the substrate deposited with the AlN material layerand then placing the substrate in a carrier and transferring the carrierto the chemical vapor deposition (CVD) chamber; step 4: depositing anitride material layer on the surface of the AlN material layer by theCVD method; step 5: depositing a highly doped active layer on thesurface of the nitride material layer; the doping concentration shouldbe as high as sufficient to improve the voltage characteristics of thelight emitting diode; step 6: depositing a p-type layer on the surfaceof the active layer with high- concentration doped impurity.

In some embodiments, the deposition method used in step 3 through step 6is the metal-organic chemical vapor deposition (MOCVD) method.

In some embodiments, the substrate is a patterned substrate with theheight of pattern of 2-20 μm.

In some embodiments, the pattern on the substrate is prepared by dryetching or wet etching or the combination of the two etching methods.

In some embodiments, the nitride material layer in step 4 is a combinedlayer of an undoped gallium nitride material layer and the n-typegallium nitride material layer.

In some embodiments, the nitride material layer in step 4 is a combinedlayer of a low-temperature gallium nitride layer, a high-temperatureundoped gallium nitride layer and the n-type gallium nitride materiallayer.

In some embodiments, the growth temperature of the low-temperaturegallium nitride layer ranges from 200° C. to 900° C.

In some embodiments, the thickness of the nitride material layer in step4 ranges from 1.0 μm to 3.5 μm.

In some embodiments, the thickness of the whole epitaxial layer of thenitride light emitting diode is less or equal to 4 μm.

In some embodiments, the high doped active layer deposited in step 6 isdoped with n-type impurity, with a doping concentration higher than6×10¹⁸/cm³.

In some embodiments, the temperature of the chamber in step 2 is350-550° C.

In some embodiments, the pressure of the chamber in step 2 is 2-10mtorr.

In some embodiments, the thickness of the AlN material layer depositedin step 2 is 5-350 Å.

In the above method, the AlN thin film layer with flat surface isdeposited on the patterned substrate having a larger depth by the PVDmethod and the thin nitrogen epitaxial layer structure is deposited onthe AlN thin film layer by the CVD method, and in the structure, theactive layer is doped with high-concentration n-type impurity; the largedepth patterned substrate improves the light extraction efficiency, andthe thinner epitaxial layer reduces the stress, thus improving thewarpage of the epitaxial wafer and further the electric uniformity ofthe single wafer; at the same time, the high-concentration doped activelayer improves the voltage characteristics of the light emitting diodestructure and further improves the overall yield of light emitting diodechip.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are provided to facilitate a further understanding of theinvention and are incorporated in and constitute a part of thespecifications, and together with the embodiments of the invention, areused for interpreting the invention but not intending to limit theinvention. In addition, the figures in the drawings are descriptive andnot drawn to scale.

FIG. 1 is a schematic diagram of a light emitting diode structureaccording to some embodiments.

FIG. 2 is a schematic diagram of a light emitting diode structureaccording to Embodiment 1.

FIG. 3 is the flow diagram of the fabrication method of a nitridesemiconductor of Embodiment 1 of the present disclosure.

FIG. 4 is the flow diagram of the fabrication method of a nitride lightemitting diode of Embodiment 4 of the present disclosure.

FIG. 5 is schematic diagram of the light emitting diode structures ofEmbodiment 4 of the present disclosure.

FIG. 6 is schematic diagram of the light emitting diode structures ofEmbodiment 5 of the present disclosure.

FIG. 7 is schematic diagram of the light emitting diode structures ofEmbodiment 6 of the present disclosure.

DETAILED DESCRIPTION

The embodiments of the present disclosure are described in details belowwith reference to the drawings.

Embodiment 1

As shown in FIGS. 1-3, a fabrication method of nitride semiconductor,comprising the following steps:

Step 1: providing a substrate 1 which can be a sapphire substrate or asilicon substrate or a patterned substrate, and placing the substrate inthe physical vapor deposition (PVD) chamber, and then depositing an AlNlayer with thickness of 5 Å-350 Å on the substrate 1 by the PVD methodto form the first buffer layer 21;

Step 2: placing the substrate on which the first buffer layer 21 isdeposited in the chemical vapor deposition (CVD) chamber, and depositingan Al_(x)In_(y)Ga_(1-x-y)N (0<x≦1, 0≦y≦1) layer with a thickness of 5Å-1500 Å by the CVD method, and then adjusting the composition of Al tohave the lattice constant between that of the AlN layer and the galliumnitride series layer; and the Al_(x)In_(y)Ga_(1-x-y)N (0<x≦1, 0≦y≦1)layer is the second buffer layer 22 with growth temperature of 400-1150°C.; combining the second buffer layer 22 with the first buffer layer 21to form the underlayer 2;

Step 3: adjusting the growth parameters such as temperature and gas flowin the CVD chamber in step 2,and then depositing an n-type galliumnitride layer 3, a light emitting layer 4 and a p-type gallium nitridelayer 5 by the CVD method on the underlayer 2, wherein, the n-typegallium nitride layer 3 is a combined layer of the undoped galliumnitride layer 31 and the n-type doped gallium nitride layer 32 in turn;in addition, the n-type gallium nitride layer 3 may also be an n-typedoped gallium nitride layer 32 (as shown in FIG. 2).

In the present embodiment, if the n-type gallium nitride layer 3, thelight emitting layer 4 and the p-type nitride layer 5 are deposited inthe CVD chamber directly in step 3 after the first buffer layer isdeposited by the PVD method, the crystal state of deposited thin film isquite different since the depositional environments of the PVD chamberand the CVD chamber are different, and the lattice coefficients of theAN layer material and the subsequent nitride layer material are greatlydifferent, which easily leads to the formation of a certain stressbetween the underlayer 2 and the subsequent gallium nitrogen serieslayer 3 and further affects the overall quality and performance of thelight emitting diodes. However, when the second buffer layer 22 composedof Al_(x)In_(y)Ga_(1-x-y)N material is inserted, since the differencebetween the lattice coefficients of the Al_(x)In_(y)Ga_(1-x-y)N materialand the material of the AlN layer and the gallium nitride layerdecreases and the lattice matching degree increases, and that the secondbuffer layer 22 and the subsequent layers are deposited in the CVDchamber, so with low difference in terms of the deposition mode, thestress between the n-type gallium nitride layer 3 and the subsequentlayers and the AlN layer is decreased and the overall crystal quality isimproved.

Embodiment 2

The difference between Embodiment 2 and Embodiment 1 is that: the firstbuffer layer and the second buffer layer contained in the underlayer 2are doped with n-type impurity, preferably silicon impurity, with adoping concentration of around 10¹⁷-10²⁰/cm³.

Embodiment 3

The difference between Embodiment 3 and Embodiment 1 is that: the firstbuffer layer and the second buffer layer contained in the underlayer 2are doped with p-type impurity, preferably magnesium impurity, with adoping concentration of around 10¹⁷-10²⁰/cm³ .

Embodiment 4

As shown in FIGS. 4 and 5, a substrate 1, which is a flat substrate or apatterned substrate and is composed of sapphire, silicon, siliconcarbide, gallium nitride, gallium arsenide or the like, is provided,wherein, the vertical height of the pattern in the patterned substrateis 2-20 μm, and the pattern can be formed by dry etching or wet etching;the large depth substrate 1 is provided in the PVD chamber, the chambertemperature is adjusted to 350-550° C., the pressure 2-10 mtorr, andthen an AlN material layer 2 with flat surface and a thickness of 5 to350 Å is deposited by the PVD method on the surface of the substrate 1by the PVD method; due to the film forming characteristics of the PVDmethod, even when the depth of the pattern on the patterned substrate ishigher than that of the conventional substrate, the AlN material layer 2can still maintain the characteristics of flat surface and high filmquality; subsequently, the substrate on which the AlN material layer 2is deposited is taken out, placed in a carrier and then transferred intothe chemical vapor deposition (CVD) chamber, the temperature of thechamber is adjusted to 400-1150° C., and then a nitride material layer 3is deposited by the CVD method on the surface of the AlN material layer2; further a highly doped active layer 4 and a p-type layer 5 aredeposited on the nitride material layer 3, the highly doped active layer4 is doped with an n-type impurity, with a doping concentration ofgreater than 6×10¹⁸/cm³, sufficient to improve the voltagecharacteristics of the light emitting diode; the deposition mode of thenitride material layer 3 through the p-type layer 5 is preferably themetal organic chemical vapor deposition (MOCVD). The light emittingdiode in such structure has a good crystal quality and little effect onthe quality of the subsequently deposited material layer since that theAlN material layer is plated by the PVD method, therefore, thesubsequent active layer can be doped with high-concentration impuritieswithout significantly reducing the crystal quality, thus avoiding theelectricity worsening such as the increase of leakage current, on theother hand, the high-concentration doped active layer structure caneffectively reduce the voltage of the light emitting diode and improvethe yield of the light emitting diode chip.

Embodiment 5

As shown in FIG. 6, a substrate 1, which is a flat substrate or apatterned substrate and is composed of sapphire, silicon, siliconcarbide, gallium nitride, gallium arsenide or the like, is provided,wherein, if the patterned substrate is selected, the vertical height ofthe pattern is 2-20 μm, and the pattern can be formed by dry etching orwet etching; the large depth substrate 1 is provided in the PVD chamber,the chamber temperature is adjusted to 350-550° C., the pressure 2-10mtorr, and then an AlN material layer 2 with flat surface and athickness of 5 to 350 Å is deposited by the PVD method on the surface ofthe substrate 1 by the PVD method; due to the film formingcharacteristics of the PVD method, even when the depth of the pattern onthe patterned substrate is higher than that of the conventionalsubstrate, the AlN material layer 2 can still maintain thecharacteristics of flat surface and high film quality; subsequently, thesubstrate on which the AlN material layer 2 is deposited is taken out,placed in a carrier and then transferred into the chemical vapordeposition (CVD) chamber, the temperature of the chamber is adjusted to900-1150° C., and then a nitride material layer 3 is deposited by theCVD method on the surface of the AlN material layer 2, and the layer isformed by combining a high-temperature undoped gallium nitride materiallayer 31 and an n-type gallium nitride material layer 32, wherein, thethickness of the undoped gallium nitride material layer 31 is 0-1.5 μm;that of the n-type gallium nitride material layer 32 is 1.0-3.0 μm; andthat of the nitride material layer is 1.0-3.5 μm; further a doped activelayer 4 and a p-type layer 5 are deposited on the nitride material layer3, the former having a doping concentration sufficient to improve thevoltage characteristics of the light emitting diode; further, thedeposition mode of the nitride material layer 3 through the p-type layer5 is preferably the metal organic chemical vapor deposition (MOCVD); thethickness of the whole epitaxial layer of the light emitting diode isless than or equal to 4 μm; the light emitting diode in such structuredue to thinner underlayer and general lower thickness has decreasedlattice stress, thus decreasing the warpage of the epitaxial wafer, andkeep consistent growth condition and electrical property for the wholesignal epitaxial wafer, and the probability of the splinters incurred inthe subsequent process; meanwhile, the large depth of the substrate usedin the structure can effectively improve the light extraction efficiencyand further improve the growth yield of the light emitting diode.

Embodiment 6

As shown in FIG. 7, the present embodiment is optimized on the basis ofEmbodiment 5, that is, when the substrate on which the AlN materiallayer 2 is deposited is taken out and placed in a chemical vapordeposition (CVD) chamber, the temperature of the chamber is adjusted to200-900° C., then a low-temperature gallium nitride layer 30 is firstdeposited on the surface of the AlN material layer 2 by the CVD method,with a thickness of 5 Å-1500 Å, and next the chamber temperature isfurther increased to 900° C. or more, and a high-temperature undopedgallium nitride layer 31 is deposited, followed by the deposition of then-type gallium nitride layer 32, and then the chamber temperature isadjusted properly before the deposition of the doped active layer 4 andthe p-type layer 5.

In this embodiment, the low-temperature gallium nitride layer 30 isfirst deposited on the AlN material layer, and then before thehigh-temperature undoped gallium nitride layer is deposited after atemperature rise, is subjected to an elevated temperature annealingtreatment so to have the low-temperature gallium nitride layer 30 toform an “island-like structure” and realize the “nucleation” process;since low-temperature gallium nitride layer 30 is grown at a lowtemperature, part of its crystal characteristics is similar to that ofthe AlN material layer 2, and part of the material properties is closeto the subsequent nitride material layer 3, the layer can well connectthe AlN material layer and the high temperature gallium nitride materiallayer, serve as a buffer to reduce the lattice stress between the AlNmaterial layer 2 and the nitride material layer 3 and further improvethe lattice quality of the subsequent epitaxial layer.

All references referred to in the present disclosure are incorporated byreference in their entirety. Although specific embodiments have beendescribed above in detail, the description is merely for purposes ofillustration. It should be appreciated, therefore, that many aspectsdescribed above are not intended as required or essential elementsunless explicitly stated otherwise. Various modifications of, andequivalent acts corresponding to, the disclosed aspects of the exemplaryembodiments, in addition to those described above, can be made by aperson of ordinary skill in the art, having the benefit of the presentdisclosure, without departing from the spirit and scope of thedisclosure defined in the following claims, the scope of which is to beaccorded the broadest interpretation so as to encompass suchmodifications and equivalent structures.

1. A fabrication method of a nitride light emitting diode, the methodcomprising: step 1: providing a substrate, and depositing an AlN layerover the substrate by physical vapor deposition (PVD) to form a firstbuffer layer; step 2: depositing an Al_(x)In_(y)Ga_(1-x-y)N (0<x≦1,0≦y≦1) layer over the AlN layer by chemical vapor deposition (CVD) toform a second buffer layer; wherein the first buffer layer and thesecond buffer layer form an underlayer; and step 3:depositing an n-typegallium nitride layer, a light emitting layer and a p-type galliumnitride layer over the underlayer by CVD.
 2. The method of claim 1,wherein said depositing in the step 2 and the step 3 is a metal-organicchemical vapor deposition (MOCVD).
 3. The method of claim 1, wherein: athickness of the first buffer layer ranges from 5 Å to 350 Å.
 4. Themethod claim 1, wherein: a thickness of the second buffer layer rangesfrom 5 Å to 1500 Å.
 5. The method of claim 1, wherein: a growthtemperature of the second buffer layer ranges from 400° C. to 1150° C.6. The method of claim 1, wherein: the underlayer is undoped or dopedwith an n-type or p-type impurity.
 7. The method of claim 6, wherein: aconcentration of the n-type or p-type impurity ranges from10¹⁷˜10²⁰/cm³.
 8. A fabrication method of a nitride light emittingdiode, the method comprising: step 1: providing and placing a substratein a physical vapor deposition (PVD) chamber; step 2: depositing an AlNmaterial layer over the substrate by PVD; step 3: moving the substrate,over which the AlN material layer is deposited, to a chemical vapordeposition (CVD) chamber; step 4: depositing a nitride material layerover a surface of the AlN material layer by CVD; step 5: depositing overa surface of the nitride material layer a highly-doped active layer withsufficient impurity to improve a voltage characteristics of the lightemitting diode; and step 6: depositing a p-type layer over a surface ofthe highly-doped active layer.
 9. The method of claim 8, wherein: saiddepositing in step 4 through step 6 is a metal-organic chemical vapordeposition (MOCVD).
 10. The method of claim 8, wherein: the substrate isa patterned substrate with a height of pattern of 2-20 μm.
 11. Themethod of claim 8, wherein: the highly-doped active layer deposited instep 5 is doped with n-type impurity, with a doping concentration higherthan 6×10¹⁸/cm³.
 12. The method of claim 8, wherein: the nitridematerial layer in step 4 is a combined layer of a high-temperatureundoped gallium nitride layer and an n-type gallium nitride materiallayer.
 13. The method of claim 8, wherein: the nitride material layer instep 4 is a combined layer of a low-temperature gallium nitride layer, ahigh-temperature undoped gallium nitride layer and an n-type galliumnitride material layer.
 14. The method of claim 13, wherein: a growthtemperature of the low-temperature gallium nitride layer ranges from200° C. to 900° C.
 15. The method of claim 13, wherein: a thickness ofsaid low-temperature gallium nitride layer is 5 Å-1500 Å.
 16. The methodof claim 13, wherein: a thickness of the nitride material layer rangesfrom 1.0 μm to 3.5 μm.
 17. The method of claim 8, wherein: a thicknessof a complete epitaxial layer of the nitride light emitting diode isless than or equal to 4 μm.
 18. The method of claim 8, wherein: atemperature of the chamber in step 2 is 350-550° C.
 19. The method ofclaim 8, wherein: a pressure of the chamber in step 2 is 2-10 mtorr. 20.The method of claim 8, wherein: a thickness of the AlN material layerdeposited in step 2 is 5-350 Å.